The invention relates generally to nondestructive methods for inspecting steel pipelines for material-loss defects such as corrosion or pits. More particularly, the invention is a system and method that uses remote-field eddy current inspection techniques and a uniquely configured excitation coil for inline inspection of pipelines having valves and other fittings that severely restrict or prevent the use of conventional inspection pigs. The invention relies on a unique collapsible excitation coil and a collapsible sensor array that enables an inspection pig incorporating the invention to pass pipeline obstructions by reducing its diameter to enable it to pass the obstruction.
Pipelines are used extensively in the United States for transporting oil and gas products because of cost, safety and efficiency considerations. Many of these pipelines have been operating for decades. Many long pipelines are buried underground or on an ocean floor, making the cost of repair and maintenance high compared to above ground pipelines. Detection of incipient defects and flaws in these pipelines becomes imperative to prevent environmental damage and costs associated with lost product and pipeline emergency repair.
Detection and characterization of pipeline defects due to material loss, such as corrosion, pits, notches, gouging, etc., are important because of the danger, expense and damage that may result from a loss of integrity of the pipeline due to these defects. Magnetic flux leakage (MFL) is the most widely used inspection technique for in situ pipeline inspection and detection of these defects. MFL inspection pigs use a circumferential array of MFL detectors and strong exciter magnets to magnetize a pipe wall to near magnetic flux saturation density. Material loss in the pipe wall due to defects such as corrosion and pits result in magnetic flux leakage near the pipe wall surface that is detected by the MFL sensors. MFL technology is normally implemented in an inspection pig that travels through a pipeline, propelled by fluid flow.
A limitation of MFL inspection pigs is a result of the structure of the MFL magnet configuration within the inspection pig. The MFL technique requires an array of powerful excitation magnets to magnetize a pipe wall to near saturation of magnetic flux density, most commonly oriented in a direction that is parallel to the longitudinal axis of the pipe. This requires magnets that are large and bulky in order to produce a magnetic field strong enough to approach magnetic flux saturation density. Using this technique, sensors are normally positioned such that they are within this region of a strong magnetic field created by the excitation magnets. Because of the length and bulk required of these magnets, it is difficult to implement a configuration of MFL excitation magnets that is sufficiently collapsible to enable an MFL inspection pig to traverse obstructions such as valves and other fittings within the pipeline. Because of these obstructions, it is not possible to inspect these encumbered pipelines with MFL inspection pigs.
Remote-field eddy current (RFEC) sensing is another established nondestructive testing method that has also been applied to inspection of pipelines. An implementation of this technique is normally located within a pipe, providing a means for detecting defects of pipe wall material on both sides of a pipe wall due to material loss, such as corrosion, pits, notches, gouging, etc. RFEC is a unique form of eddy current testing of pipelines that uses an electromagnetic excitation coil positioned within a pipe such that it is oriented coaxially with the pipeline axis. RFEC excitation coils do not have to produce the magnetic field approaching magnetic flux saturation density as required by MFL magnets. RFEC sensors are located inside the pipe, but are positioned several pipe diameters distance from an excitation coil. This RFEC configuration provides sensors that are in a “remote-field” zone of the magnetic field produced by the coaxial RFEC excitation coil, where direct electromagnetic coupling from the excitation is minimal. The RFEC sensors detect a magnetic field that originated at the excitation coil, penetrated through the pipe wall to the outside diameter, and re-entered the pipe wall to the inside diameter at the sensor location. Since the magnetic field has penetrated the pipe wall, it is strongly affected by the pipe wall thickness, providing detection of defects that result in material loss that changes the pipe wall thickness.
RFEC technology is not routinely applied to pipeline inspection because the MFL method is more straightforward and can have a greater sensitivity. However, for pipelines that have obstructions that prevent MFL inspection pigs from traversing the pipeline, the configuration of an RFEC inspection pig offers an approach that allows an inspection pig to traverse obstructions in the pipeline.